1. Field of the Invention
This invention pertains to printing machines. More particularly, this invention pertains to a printing machine having infrared sensing to control positioning of a tape.
2. Description of the Prior Art
An example of a printing machine is shown in commonly assigned U.S. Pat. No. 4,815,871 dated Mar. 28, 1989. Such printing machines include a tape and a ribbon contained within a removable cassette. The cassette is mounted to the machine. Internal circuitry within the machine advances the tape past a printing head.
In the machine of the '871 patent, the printing head is a thermal printing head having a plurality of individually activated locations referred to as "pixels". The pixels oppose a drive roller or platen. The ribbon and the tape are positioned between the pixels and the drive roller in face-to-face abutting relation.
The drive roller advances both the tape and the ribbon in steps of discrete lengths of travel. After each step there is a pause during which the pixels are energized to heat causing transfer of ink from the ribbon to the tape, corresponding with the energized pixel locations. After such transfer of ink, the tape and ribbon are again incrementally advanced and the same or different pixels are energized to cause an additional transfer of ink. After successive advancement of the tape and the ribbon and successive energization of different pixels, a complete image (for example, a letter of the alphabet) is formed on the tape. In this manner, an entire message is printed.
The machine includes a keyboard which permits an operator to input information regarding the message to be printed. Also, such machines may have jack locations for permitting direct connection of the machine to a personal computer or other device such that information on the message to be printed is transferred directly from the personal computer to the circuitry of the printing machine, which then controls operation of the tape drive and print head.
The individual cassettes used in the printing machine may contain circuitry which permits identifying characteristics regarding the cassette and its contents to be interfaced with the circuitry of the print machine. For example, a tape cassette of the prior art may contain a resistor or other circuit element. The particular electronic characteristics of the element are selected to correspond with the tape contained within the cassette. By way of example, a resistor of a predetermined ohms may indicate that the cassette is carrying a white tape for receiving a black image.
Cassettes with identifying information have become progressively more sophisticated. An example of a more sophisticated cassette is shown in U.S. Pat. No. 5,318,370. In that patent, a tape cassette is shown which includes a memory circuit component which may contain a wide variety of information regarding the cassette. For example, the memory component may contain in its memory such information as the size, type, burn time, length and color of the tape contained by the cassette. Further, as illustrated in that patent, when the cassette is attached to the printing machine, the memory circuit component interfaces with the circuitry of the printing machine in an interactive manner. For example, as tape is advanced from the cassette, the printing machine can read into the memory circuit component the remaining length of tape on the cassette.
Frequently, printing machines are used to print images on a die-cut label contained on a tape. In a die-cut label tape, individual labels are separately positioned on a liner with the labels being spaced apart by fixed spacing on the liner. To insure accurate positioning of a desired message on a label, the tape must be in accurate alignment (i.e., in registration) with the thermal print head.
Prior art printing machines utilized light in the form of infrared energy to insure consistent registration. The printing machine of the prior art used both an infrared transmitter and an infrared receiver. The infrared beam generated by the transmitter was directed through the tape supply as it was advanced through a tape path. The amount of infrared energy passed through the tape was detected and measured by the infrared receiver. Less energy passing through the tape indicated that the beam was being directed through a layer of the tape containing both the liner and label material. High energy transmission through the tape indicated that the beam was passing through a liner layer not having a label layer. In this manner, infrared systems detect changes in the IR transmission levels and determine transitions from liner only to liner/label positions.
Infrared transmitters can vary from machine to machine. Also, the amount of infrared energy emitted from a transmitter can vary over time as the transmitter becomes dirty. In addition, there are variations in receiving sensor values which can change significantly from machine to machine. In view of these factors, a problem existed in keeping consistent registration while printing on die-cut labels. The prior art apparatus using infrared sensing requires that the end user of the machine make an electrical-mechanical adjustment to the transmitting infrared LED to change the amount of IR energy being admitted from the source in order to retune the sensitivity of the transmitter/receiver pair to acceptable levels. Unfortunately, user adjustment is both cumbersome and subject to error. It is an object of the present invention to provide an automatic calibration system for infrared sensing of labels.